Which is better at reducing/preventing stretch marks: bio oil or shea butter or other?

Myth but use if help: Many of my patients swear by the benefits of tea tree or bio-oil and so i generally don't dissuade them but there is very little science to support its use. It is probably the massage, (which we know helps scars) that is used to apply the oil, which is actually providing the improvements observed. Massage with any clean oil 3-4 times a day. Silicon gel trips are more likely to improve the scar.

No harm: Neither is likely to cause you any harm. Hydration and massage may help a little, but there is little evidence that these products are helpful in any specific way.